I think Killian and Ronan might be a bit out of place amongst your biblical quartet. Adam and Aaron would be very refreshing, and would fit right in.

I would normally endorse William right away for the personal connection to your father, but I wonder if the other four children would feel like this fifth one was somehow a bit more special, as he got granddad's name, and the only non-OT name to boot? I definitely feel that would be the case if you were to give him the name James. I think that would be an immediate set up for sibling rivalry.